1. Bakersfield, Calif.

By Bryan WalshThursday, Sept. 29, 2011

David McNew / Getty Images

It makes sense that Bakersfield owes its early boom years in the 19th century to the oil industry, because it's oil that has enabled the suburban sprawl and congestion that has caused the city to have perhaps the worst air pollution in the U.S. It's not all bad  high-ozone days in Bakersfield have dropped over the past decade (the economic slowdown may have played a role). But if the residents of Bakersfield and the rest of Southern California ever want to experience truly clean air, they're going to need to look to innovative solutions: electric cars, public transit and maybe even the occasional walk.